The present invention relates to an input device for systems employing display screens. More specifically, the present invention provides an inexpensive, optically-based overlay for a display screen which, with support electronics, functions as a touch screen input device.
There are currently four different types of commercially available touch screen technologies: infrared, resistive membrane, capacitive, and surface acoustic wave. All of these are active technologies in that they rely on generating an active signal and observing the effect on the active signal of an object against the screen, i.e., a touch event.
Infrared technology was the first type of commercially available touch screen technology. According to infrared technology, infrared beams are transmitted across the screen in a grid pattern. Upon the occurrence of a touch event, specific beams are interrupted and the location of the touch event is determined from the known intersection of the interrupted beams.
Resistive touch screen technology employs a glass or acrylic panel coated with thin, transparent, electrically conductive and resistive layers through which electrical current flows. The layers are separated by transparent separator dots. When pressure is applied to the screen, the layers are pressed together thereby affecting the resistivity of the layers at the location of the touch event. This change in resistivity is registered as a change in current from which the location of the event is determined.
Capacitive touch screen technology employs a plurality of electrodes which generate a substantially uniform electric field across the screen. In response to a touch event, the capacitance across the electrodes at the location of the event is altered thereby altering the electric field at that location. Monitoring circuitry detects the disturbance of the field and reports a touch event at the location of the disturbance.
Surface acoustic wave technology, as the name suggests, transmits acoustic waves across the surface of the display screen. As with infrared technology, the location of a touch event is determined from the interruption of the waves by the event.
All of these technologies are mature and are used to produce a wide variety of products, some of which are manufactured in reasonably high volume. However, as will be understood, each technology has its own calibration and mechanical registration issues which tend to keep the cost and complexity of touch screens high and their reliability low.
Another type of input device technology which involves the display screen and which addresses some of the issues discussed above is the light pen. In the conventional cathode ray tube (CRT) display screen, an electron gun scans a raster beam across a phosphor screen one raster line at a time starting at the upper left corner of the screen. When a raster line is complete, i.e., the raster beam reaches the edge of the screen, the electron gun turns off, moves down one line, turns on again, and scans the next line. This is repeated until the entire screen is scanned at which point the gun moves to the upper left corner of the screen to repeat the process. As the raster beam hits the phosphor on the screen, the phosphor glows brightly and then slowly dims until the beam hits it again. In the typical computer system, a display or graphics card generates the video signal which controls the raster beam according to precise timing information.
The typical light pen includes a pinpoint lens and small photodetector at the tip of the pen connected to a Schmitt trigger flip-flop. The tip of the light pen is held in contact with the display screen in response to which the light pen generates an output pulse representative of the phosphor glow and decay at the location of the contact. Based on the time at which the output pulse of the light pen is received and the raster timing information from the graphics card, circuitry to which the light pen is connected correlates the occurrence of the pulse with the known position of the raster beam at that time thereby determining the exact location of the tip of the light pen.
However, while the light pen addresses several of the issues presented by commercial touch screen technologies it is more complex from a user's perspective, adding yet another peripheral device to be connected and configured. The light pen also represents another level of complexity from a reliability perspective which is generally undesirable.
Another type of optically-based touch screen technology has been described in the literature but has not been implemented in any known commercially available products. The technology is a passive technology which, if viable, would also require no calibration or mechanical registration with respect to the display screen. As described in U.S. Pat. Nos. 4,346,376 and 4,484,179, the entire specifications of which are incorporated herein by reference for all purposes, and as shown in FIGS. 1a and 1b of the present application, a transparent glass or plastic plate 102 is placed in front of the CRT 104. At the edge of the plate, one or more detectors 106 are mounted to receive light 108 from CRT 104 which is diffused by an object (e.g., a fingertip 110) in contact with plate 102 and trapped within plate 102 by total internal reflection, i.e., diffused by the object beyond the critical angle of the plate material.
As with the light pen technology discussed above, the reception of the light corresponding to the touch event is intended to be correlated with the raster timing information to determine the location of the event. However, unlike the detector in the light pen which receives a significant portion of the light emitted by the CRT at the touch location, the detectors in this optically-based technology receive a very small fraction of the light diffused by the fingertip and are simultaneously exposed to significant and undesirable light energy from other sources. For example, parasitic reflections of light from the display screen which are within the critical angle reach the detectors resulting in a very poor signal-to-noise ratio. This presents extremely challenging requirements for the touch event detection circuitry as described in U.S. Pat. Nos. 4,305,071 and 4,707,689, the entire specifications of which are incorporated herein by reference for all purposes. Thus, while the previously described optically-based touch screens have some desirable characteristics, there continue to be significant challenges to the commercial viability of the technology.
Another optically-based touch screen is described in U.S. Pat. No. 4,868,551, the entire specification of which is incorporated herein by reference for all purposes. The principle on which this input device is based is the same as that relied on in the '376 and '179 patents and discussed above with reference to FIGS. 1a and 1b. The devices described in the '551 patent attempt to solve the signal-to-noise problem by increasing the efficiency with which its photodetectors collect light. This is done through the use of concentrating or focusing systems in conjunction with reflective coatings at some edges of the touch plate. Some of these focusing systems comprise extensions of the plate material having geometries which, with the use of reflective coatings, tend to concentrate the light energy trapped within the plate toward a photodetector disposed at a narrow end of the extension.
Unfortunately, even if the focusing extensions and reflective coatings work as described, the problem of parasitic reflections and their effect on the signal-to-noise ratio is not addressed. That is, the photodetectors of the '551 patent would still not be able to distinguish between light energy corresponding to a touch event and undesirable parasitic reflections.
It is therefore desirable to develop improvements to previously described optically-based touch screen technologies which increase the collection of light corresponding to a touch event and simultaneously reduce the amount of parasitic noise received, thereby improving the signal-to-noise ratio.